Hearing systems which rely on amplified acoustic drive output are well known in the prior art. For example, public address (PA) systems used in auditoriums or concert halls provide sounds to an audience with normal hearing by widespread production of amplified acoustic information in the form of sound waves. Such systems, however, are incapable of selectively imparting audio information to some nearby individuals but not to others. If amplification of such systems is increased so as to enable hearing impaired individuals to receive the information, the volume may be too loud for persons with normal hearing.
Another type of acoustic drive sound system is exemplified by the commonly available acoustic hearing aid. Such devices rely on acoustic output provided by a miniature speaker typically located within the wearer's external ear or ear canal, and positioned adjacent to the ear drum. In many cases, a portion of the acoustic output feeds back to the input of the device, causing a self-sustained oscillation. This "feedback" phenomenon, which is generally proportional to the magnitude of the gain, imposes limitations on the amount of gain available to the wearer. As a result, many hearing impaired individuals cannot benefit from such devices. This is especially true for severely hearing impaired individuals, where high acoustic gain requirements result in unacceptable levels of acoustic feedback.
Some prior art hearing systems have utilized electromagnetic energy to vibrate the middle ear structures or the tympanic membrane. An example of this may be found in U.S. Pat. No. 4,957,478 to Maniglia. The Maniglia system uses a hearing device consisting of a microphone, an amplifier, a power source, and an electromagnetic coil placed in the external auditory canal for receiving sound waves and converting them into magnetic fields. A permanent magnet is surgically implanted onto a portion of the bones of the ossicular chain of the individual. The magnet responds to the applied magnetic field, and causes the bones of the ossicular chain to vibrate with the same frequency and amplitude variation as the incoming signal at the microphone. In an alternate embodiment, a sound signal is externally modulated with a radio frequency (RF) signal. The signal is then transmitted through an external coil worn behind the ear to a coil implanted in the mastoid cavity. The mastoid cavity coil decodes the signal to retrieve the original signal, which is then applied to a third coil which vibrates the implanted magnet, resulting in the perception of sound. In the above cases, as well as others not mentioned here, a considerable amount of conspicuous hardware is required for the system to operate. Additional disadvantages include surgical intervention and the various associated risks.
Another type of system proposed by the prior art (Rutschmann, 1959; Goode, 1973) utilizes a small magnet glued or otherwise attached to the tympanic membrane. A coil placed inside or just outside the external auditory canal is driven by currents to produce electromagnetic fields which vibrate the magnet attached to the tympanic membrane. In the case of the Rutschmann system, the current requirement was impractically high for hearing thresholds. The Goode system, on the other hand, provided some improvements in terms of power consumption, but the requirements were still impractically high. Power consumption and other limitations related to coil and magnet design made it necessary, in those systems, to place the coils in close proximity to the magnet.
An improved electromagnetic drive hearing system is shown and described in U.S. patent application Ser. No. 679,661, filed Apr. 1, 1991 and assigned to the assignee of the present invention. These systems permit the use of an inconspicuous in ear magnet transducer driver by a coil which may be positioned inside or outside the ear canal of an individual.
If a system such as those shown and described in the aforementioned patent applications is to be powered by a battery, it is important to minimize the drain on the battery. In this way, lightweight batteries may be used without having to be replaced at inconveniently frequent intervals. Systems for driving electromagnetic type hearing systems and which are currently known in the prior art do not lend themselves to accomplishing this goal. Such prior art systems, which require relatively large currents, have not utilized circuitry with power requirements suitable for the use of relatively lightweight batteries.
Hence it would be desirable for a system to be able to vary a magnetic field in response to ambient sounds in such a way that the energy required to maintain and modulate the electromagnetic field would be provided by lightweight batteries with long usage lives. Such a system should produce an electromagnetic field large enough to communicate with a remote magnetic transducer while utilizing low power.
Broadly, it is an object of the present invention to provide an improved hearing system.
It is a further object of the present invention to provide a hearing system which can communicate with a magnetic transducer by means of a modulated electromagnetic field for long periods while using small current sources.
It is a further object of the present invention to provide a hearing system as described which utilizes low power and communicates by inconspicuous means with a magnetic transducer.
These and other objects of the present invention will be apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.